Materials interposed between contacting dental surfaces, such as occluding dental arches, tend to interfere with the proper contact of the surfaces. Characteristics inherent with these materials, such as thickness and resistance to deformation, prevent the dental surfaces from contacting properly.
For occluding surfaces, the materials further interfere with proper contact of the teeth by triggering neuromuscular proprioceptive responses, which tend to pull the jaw away from centric occlusion when interferes are encountered.
Procedures which utilize materials interposed between contacting dental surfaces include interference marking, occlusal registration, dual arch impressions for indirect restorations, molding of composites, and so on.
For the descriptions contained herein, general references to “teeth” include any dental restorations or prostheses thereof, including but not restricted to: fillings, inlays, onlays, veneers, crowns, bridges, partial or full denture teeth, and restored implants. General references to “appliances” include guards, splints, stents, retainers, and partial or full dentures, and so on.
Interference Marking
Interfering contacts between teeth, restorations, or appliances, are typically disclosed by marking with inked articulating paper. It is intended that the contact pressure transfers the ink onto the teeth.
However, inks and papers tend to create many false positive and false negative marks because they mark where they shouldn't, and don't mark where they should. In addition, the thickness of the paper can interfere with proper contact, such as occlusion, or seating restorations and appliances. As a result, dentists waste time trying to locate the actual interferences, and sometimes inadvertently relieve valuable dental structure.
Other marking systems involve painting ink or indicating paste to the surfaces of the teeth, restorations, or appliances, and the contact pressure and friction then removes the ink. These are subject to problems similar to those of the ink papers.
Another method of marking interferences involves interposing a PTFE strip between the luting surface of an indirect restoration, including crowns, inlays, and onlays, and the prepared surfaces of a tooth. However, general use PTFE strips are fragile and difficult to use. They are also less reliable as an interference indicator when not adhered, due to inadvertent repositioning with respect to the dental surface upon which it rests.
Molding Restorative Materials
Multiple methods are used to shape hardenable restorative materials, such as composites, in cavity preparations which oppose appliances.
For a first method, the composite is hardened first. The composite is then fitted to the appliance by trial and error abrasion. This method is time consuming, and the fit of the composite to the appliance is frequently poor.
For a second method, the appliance is seated against the composite prior to hardening, so the appliance directly shapes the composite. The fit of the composite to the appliance is good. However the composite sometimes locks the appliance onto the restored tooth.
For a third method, a plastic sheet is interposed between the unhardened composite and the appliance such that the composite is shimmed away from locking undercuts. However, the plastic sheet is likely to interfere with proper seating of the appliance, and tends to form an irregular composite surface.
Occlusal Registration
Occlusal registrations record the locations, and sometimes the relative force, of occlusal contact areas of the teeth. Occlusal registrations are used for the fabrication of crowns, bridges, prostheses and appliances, as well as record the patient's occlusal status during occlusal therapy or TMJ therapy.
Materials currently used for occlusal registration include impression materials, wax wafers, and electronic sensors. While the impression materials typically do not interfere with proper occlusion, they are costly, and long-term storage is difficult due to their fragility and bulkiness. Some waxes can cause occlusal interference, and are difficult to store due to temperature sensitivity and fragility. While electronic sensors permit digital storage, and indicate relative occlusal contact forces in the z-axis, they are costly, and can interfere with proper occlusion.
Dual Arch Impressions
Trays used for dual arch impressions have a peripheral frame connected to a thin central membrane for supporting impression material. With one impression sequence, dual arch trays acquire impressions of either arch, as well as an occlusal registration.
The membrane is generally comprised of gauze materials of minimal thickness and rigidity, such that interference to the occlusion of the teeth is minimized. However, the membrane causes a degree of interference to proper occlusion.
Two Phase Impressions
The detail captured by dental impressions is improved when a closely fitting tray increases the hydraulic pressure of the impression material, and thereby drives the impression material into smaller spaces. A method for forming closely fitting trays involves taking a preliminary, or phase one, impression prior to the final, or phase two, impression. The phase one impression can thereby function as a closely fitted custom tray for the phase two impression.
A first method for forming a phase one impression is to interpose a plastic sheet between the impression material and the teeth before seating the impression tray over the teeth. The sheet reserves a small space over the surface of the phase one impression. During the phase two impression, the reserved space reduces interferences to reseating the phase one impression back over the teeth. The reserved space also provides an escape for the flowing phase two impression material displaced by the occluding teeth. This reduces excessive pressure of the flowing phase two impression material against the set phase one impression material, and thereby reduces distortion of the phase one impression material. Substantial distortion of the phase one impression during the phase two impression can in turn elastically distort the phase two impression after removal from the teeth.
Spacing phase one impressions with the plastic spacer sheet method is generally used for single-arch impression trays only. However, interposing a substantially non-deformable plastic sheet between the teeth and the impression material on a dual arch tray would interfere with the proper occlusion of the teeth.
A second method for forming a phase one impression is to manually relieve the surface of the phase one impression with a large bur, to create space for the phase two impression material. However, this is time consuming and inexact.
A third method for forming a phase one impression is to make the phase one impression having no relief spacing, but create a hole through the impression in the vicinity of the prepared tooth. The phase two impression is formed by partly loosening the phase one impression from the teeth, injecting phase two impression material through the hole, and reseating the teeth into the injected impression material until cured.